The present invention relates to a hue adjustment circuit used in TV sets.
In NTSC TV sets, a hue adjustment (TINT) circuit is nearly indispensable in order to correct a tint shift caused when demodulating a chroma signal.
On the other hand, devices such as digital video disks (DVDs) have spread in recent years. In order to connect these devices to a TV set, capability of inputting a YUV signal (color difference signal) besides a composite video signal to a TV set is being demanded.
When the YUV signal is input, it is not demodulates and consequently a hue adjustment circuit should be essentially unnecessary. For the purpose of delicate tint adjustment, it is also requested to hue adjust the YUV signal (color difference signal) as well.
The present invention aims at implementing such a hue adjustment on the color difference signal using a compact configuration.
The conventional art will now be described.
FIG. 2 is a diagram showing the conventional art.
In FIG. 2, numeral 1 denotes a 90xc2x0 phase shift circuit for shifting the phase of a sub-carrier signal (hereafter referred to as fsc signal) by 90xc2x0, 2 a TINT phase shift circuit for shifting the phase of the fsc signal in order to adjust the hue, 3a a circuit for multiplying the fsc signal subjected to phase adjustment in the TINT phase shift circuit 2 by the fsc signal subjected to phase shift in the 90xc2x0 phase shift circuit 1.
In FIGS. 2, 4a, 4b, 4c and 4d are circuits for multiplying output signals of multiplier circuits 3a and 3b by a color difference signal (a Rxe2x80x94Y signal and a Bxe2x80x94Y signal), and 5a and 5b are circuits for addition and subtraction of outputs of the multiplier circuits 4a, 4b, 4c and 4d. 
FIG. 3 is a diagram showing a vector (hue) of a chroma signal.
FIG. 4 is a vector diagram showing phase relations among a burst signal, an original fsc signal FSC(0), a fsc signal FSC(90) subjected to phase shift in the 90xc2x0 phase shift circuit 1, and a fsc signal subjected to phase adjustment FSC(xcex1) subjected to phase adjustment in the TINT phase shift circuit 2.
In FIG. 3, it is provisionally assumed that the vector (hue) of a chroma signal of a certain color is A and its angle is xcex8. The angle of the burst signal is 180xc2x0.
When a chroma signal having a hue A is multiplied by the fsc signal and thereby demodulated to the color difference signal (Rxe2x80x94Y and Bxe2x80x94Y), the Rxe2x80x94Y signal and the Bxe2x80x94Y signal can be represented as sin xcex8 and cos xcex8, respectively.
When the hue A is changed to a hue Axe2x80x2 by changing the phase by xcex1, the color difference signal Rxe2x80x94Y and Bxe2x80x94Y demodulated by the fsc signal become sin (xcex8+xcex1) and cos (xcex8+xcex1).
From the addition theorem, the following equations (1) and (2) are derived.
sin (xcex8+xcex1)=sin xcex8 cos xcex1+ cos xcex8 sin xcex1xe2x80x83xe2x80x83(1) 
cos (xcex8+xcex1)=cos xcex8 cos xcex1xe2x88x92 sin xcex8 sin xcex1xe2x80x83xe2x80x83(2) 
Therefore, hue adjustment can be conducted by arithmetic operations on the color difference signals Rxe2x80x94Y (sin xcex8) and Bxe2x80x94Y (cos xcex8) and hue correction components sin xcex1 and cos xcex1.
The hue correction components sin xcex1 and cos xcex1 can be generated from the fsc signal by using the 90xc2x0 phase shift circuit 1 and the TINT phase shift circuit 2 as shown in FIG. 2.
Hereafter, its principle will be described.
Typically, demodulation from the chroma signal to the color difference signal (Rxe2x80x94Y signal and Bxe2x80x94Y signal) is conducted by multiplying two fsc signals, synchronized in phase to the burst signal and differing in phase by 90xc2x0, by the chroma signal.
The two fsc signals differing in phase by 90xc2x0 are obtained from the original fsc signal (hereafter abbreviated to FSC(0)) and a fsc signal (hereafter abbreviated to FSC(90)) shifted in phase by 90xc2x0 from FSC(0) in the 90xc2x0 phase shift circuit 1. FIG. 4 shows phase relations among the burst signal, FSC(0), FSC(90).
Here, in the TINT phase shift circuit 2, the phase of FSC(0) is changed by xcex1. The output of the TINT phase shift circuit 2 is FSC(xcex1) . FIG. 4 shows phase relations among the burst signal, FSC(0), FSC(90), and FSC(xcex1).
If FSC(0) is multiplied by FSC (xcex1) in the multiplier circuit 3a of FIG. 2, a component cos xcex1 a is obtained from its output.
In the same way, if FSC(90) is multiplied by FSC(xcex1) in the multiplier circuit 3b of FIG. 2, a component sin xcex1 is obtained from its output.
Results of these multiplication operations are evident from FIG. 4 as well. Since cos xcex1 and sin xcex1 are scalar quantities obtained from multiplication of vectors, it is understood that they are direct current signals having neither frequency components not phase components.
As described above, the hue correction components sin xcex1 and cos xcex1 can be obtained from the fsc signal.
On the other hand, the multiplier circuits 4a and 4b, the adder circuit 5a, and the subtracter circuit 5b are circuits for implementing the equations (1) and (2).
As a result of computation operations conducted in the multiplier circuits 4a and 4b, the adder circuit 5a, and the subtracter circuit 5b, the Rxe2x80x94Y signal changed in hue by xcex1 (sin (xcex8+xcex1)) and Bxe2x80x94Y signal changed in hue by xcex1 (cos (xcex8+xcex1)) can be obtained.
If the conventional art is used in a TV set, however, a circuit for generating the hue adjustment components sin xcex1 and cos xcex1 is separately needed. This results in a drawback that the circuit scale becomes large and the system itself becomes complicated.
It is an object of the present invention to facilitate circuit implementation and make the circuit scale smaller by using the chroma signal demodulation circuit in the TV set as the hue adjustment circuit as well.